There is an increasing demand for the provision of fixed point-to-point, or point-to-multi-point satellite communications services. The use of the orbital frequency spectrum that meets the needs for satellite communication services has increased significantly, resulting in scarcity or unavailability of geostationary orbital slots, or satellite positions, for certain prime frequency bands, and orbit locations. The frequency spectrum used by satellites in the geostationary satellite orbit is finite and its reuse by those satellites is limited by the level of interference incurred between the different satellite systems in that orbit sharing the frequency bands. With this present level of increasing congestion (both spectrum and orbital), a need exists to establish alternative systems, where satellites have essentially the same user characteristics as satellites in the geostationary satellite orbit, in that the satellites remain almost stationary relatively to the ground, and provide, without any ground intervention, continuous service to the users.
An object of the present invention is to provide such an alternative. The orbit of this invention allows the spectrum used by satellites in the geostationary satellite orbit to be completely reused by satellites of this invention without causing any interference. Communication ground stations operating with satellites of this invention will be the same as the ground stations operating with satellites in the geostationary satellite orbit. The only difference is that, by virtue of an angular separation of 45 degrees or more between satellites of this invention and the geostationary satellites, as seen from any ground location, the ground terminal of the systems of this invention will be pointed to a different place in the sky.
Another object of the present invention is to place each satellite in an orbit that appears almost stationary relatively to a fixed point on the earth during an operational portion of the orbit, along which the satellite defines to a ground-based user a small loop, hence its characterization as "geostationary". In fact, to a ground-based observer, at a single longitudinal location, a pair of loops are defined, one being a minor loop at the highest latitude with its apogee extending to 63.4 degrees, and the other being a major loop, the pair of loops being disposed on one side of the equator. Since the operational portion of the orbit (the minor loop) is located at high latitudes (above 45 degrees N. or S. and at or below 63.4 degrees N. or S., respectively), this invention is defined as a High Latitude Geostationary Satellite System. A distinction that needs to be made between satellites operating in the orbits of this invention and more popular "Geostationary Satellites" is that satellites of this invention are geostationary during a part of their orbit, namely their operational part, whereas the popular geostationary satellites are located in a 24 hour orbit which is geostationary, and therefore the satellites are stationary for 24 hours per day.
Another object of the present invention is to achieve a switching of active communications between satellites by use of precise timing signals from the Global Positioning System (GPS) and the use of inter-satellite links between the satellites in order to ensure that a seamless transition of communication services is provided to users on the ground without the need of any ground control intervention.